The present invention relates to the field of electron beam systems and in particular to electron beam systems that produce multiple electron beams.
Because a beam of electrons can be focused to a very small spot, instruments using electron beams can be capable of very high resolution. Electron beams are used in electron microscopes for forming images of microscopic structures and in electron beam processing systems, such as electron beam lithography tools, for creating microscopic structures. For example, electron beam systems are widely used in the manufacturing of integrated circuits to create lithography masks or to create structures directly on a semiconductor wafer.
Electron beam systems typically include an electron gun as a source of electrons and an electron optical column comprised of lenses that focus and direct the electrons emitted from the electron gun. The electron gun typically includes an emitter, extractor, suppressor, and may include one or more electron optical elements. The emitter emits electrons with the help of an electric field supplied by the extractor. The suppressor suppresses emission of electrons from the sides of the emitter tip. Such electrons would not end up forming part of the beam. The one or more gun optical elements assist in focusing the electrons into a beam. Because air would disperse the electrons in the beam, the entire electron path, from the electron gun to the target, must be maintained in a vacuum, which adds to the cost and complexity of electron beam systems.
Although electron beam systems usually have higher resolutions than systems that use light, electron beam systems are typically not capable of rapidly processing a large number of integrated circuits and so have not been suitable for high volume production or inspection. One method of increasing the processing rate, or throughput, of electron beam systems is to include within a single vacuum chamber multiple electron beams that can operate on a target simultaneously. For example, U.S. Pat. No. 4,390,789 to Smith and Harte describes an electron beam lithography system that includes nine electron sources and nine electron optical columns. The system described in U.S. Pat. No. 4,694,178 to Harte includes twelve electron sources and twelve electron columns.
Similarly, U.S. Pat. No. 5,981,962 to Groves et al. describes a multiple beam system that uses a relatively large surface area, low brightness source. Although such sources are easier to handle in an array than are small, high brightness sources, a system using large area sources cannot provide as high a resolution as a system using small, high brightness sources. U.S. Pat. No. 6,023,060 to Chang et al. describes a multiple beam system that uses multiple T-shaped electron beam columns. International Patent Publication WO 99/47978 describes a method of handling a mismatch between the distance between the electron columns and the distance between dies on the target. International Patent Publication WO 98/48443 describes a multiple beam system in which the multiple beams do not have separate optical columns and in which the system electron optics operate on the multiple beams as if they were a single beam.
Multiple beam electron systems have not gained acceptance in industry because they have been unreliable and time consuming to service. In particular, the high brightness electron sources required for high resolution systems are relatively fragile and have a limited lifetime. Failure of a single electron source can cause other electron sources in the system to also fail, and replacing an electron source requires taking the entire system out of service.
One widely used, very bright electron source is a thermal field emitter known as a xe2x80x9cSchottky emission cathodexe2x80x9d or xe2x80x9cSchottky emitter.xe2x80x9d Schottky emitters typically operate at temperatures of about 1,800 K. The surface of a Schottky emitter from which the electrons are emitted is very sensitive to surface contamination and the emitter will not function properly if foreign molecules are adhered to the working surface. Before a Schottky emitter can be placed in service, it must be conditioned by a lengthy conditioning process, referring to as a xe2x80x9cbakeout,xe2x80x9d which entails baking the emitter and gun to remove adhered molecules from their surfaces.
The useful life of a Schottky emitter is much less than the expected life of the electron beam system, so Schottky emitters need to be replaced periodically. Replacing a single emitter requires opening the vacuum chamber, which exposes the other emitters in a multiple emitter to air. All the emitters then need to be conditioned again before they can be used.
When Schottky emitters are being conditioned or when they are restarted in normal use, they will intermittently eject contamination and cause contamination to be ejected from the extractor and other elements. This phenomenon, known as xe2x80x9coutgassing,xe2x80x9d results when electrons emitted from the emitter strike the extractor and other elements, causing the sudden ejection of gas molecules and other contaminants that were adsorbed onto surfaces during the period when the emitter was not operating. These gas molecules may then collide with other components in the vacuum chamber causing them to emit more gas molecules. This outgassing may increase the gas pressure in the vacuum chamber enough to precipitate emitter arcing (excessively large electron emission), which can damage the emitter tip. This arcing in turn increases the gas pressure in the vacuum chamber and can cause any other emitters present in the vacuum chamber to also arc.
The difficulty with maintaining multiple Schottky emitters in a single system has prevented the widespread use of multiple electron beam systems.
An object of the invention is to provide a reliable electron beam tool using multiple electron beams to achieve a high processing rate.
The present invention comprises an electron beam system that uses multiple electron guns within a single system. The electron guns are contained in one or more sealable vacuum chambers that can be vacuum isolated from a chamber that contains the work piece that is the target of the beams. The electron guns are preferably relatively isolated from each other during operation, so that failure of one gun is less likely to cause failure of the other guns.
In some embodiments, multiple electron guns are positioned in a single gun chamber that can be vacuum isolated from the vacuum chamber or chambers containing the target and other electron optical elements. In some embodiments, each electron gun is in an individual sealable gun chamber so that the system can be opened and any electron gun can be replaced without exposing the remaining guns to air. Gun chambers can be removed and replaced individually, or individual gun chambers can be grouped into sealable intermediate chambers that can be removed and replaced.
When an electron gun fails, the gun chamber containing the faulty gun is removed from the system and replaced with another sealed gun chamber, preferably containing one or more electron guns that have already been conditioned. Thus, the electron beam system can be put immediately back into production, without the delay of conditioning the new electron gun in the system.
Positioning electron guns in individual gun chambers reduces the conduction of gases from one gun chamber to the other, so arcing of one gun is less likely to cause the failure of other guns in the system. Even in an embodiment in which multiple guns are in a single chamber, a preferred column design serves to reduce the conduction between guns.
In a preferred embodiment, each gun chamber has its own ultra high vacuum pump. The multiple electron beams preferably are capable of operating simultaneously on different parts of a single target, such as different die of a semiconductor wafer.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed can be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.